This invention relates to down-hole decelerators for decelerating a device dropped down a borehole in a drilling mud column.
It is conventional practice to drop measuring instrumentation and other devices down the inside of a hollow drill string filled with drilling mud in order to locate the instrumentation at a position down-hole in the vicinity of the drill bit. Examples of tools which are commonly introduced into the borehole in this manner are electronic single-shot and multi-shot tools and coring tools.
Furthermore it is usual for the drill string to incorporate a landing plate to receive the tool at its intended location within the drill string. It will be appreciated that measuring instrumentation dropped down the borehole in this manner will experience a high impact load on contacting the landing plate, and this may result in damage to the measuring instrumentation and possible loss of drilling time in the event that the measuring instrument requires replacement.
Accordingly it has long been the practice for a tool which is to be dropped down-hole to be provided with a form of decelerator which is commonly referred to in the art as a stinger. Such a decelerator comprises a plunger having a nose for contacting the landing plate, and a stiff spring surrounding the plunger and located between the nose and a sleeve within which the plunger is slidable. When the nose contacts the landing plate, the plunger is displaced against the action of the spring, and accordingly the spring cushions the impact to some extent. However, a decelerator of this form is extremely inefficient, and does not provide an adequate safeguard against damage to measuring instrumentation dropped downhole.
It is an object of the invention to provide a novel form of down-hole decelerator which is considerably more efficient than this prior form of decelerator.